Systemic Lupus Erythematosus (SLE) is a systemic autoimlnune disease characterized by both B-cell and T-cell abnormalities. B-cells play a fundamental role in SLE and primary B-cell defects have been shown to result in SLE-like disease in multiple animal models. Many such defects are thought to induce breakdown of tolerance to self-antigens leading to the expansion of autoreactive B-cell populations and the production of pathogenic autoantibodies. However, B-cells also contribute to the pathogenesis of the disease by antibody-independent mechanisms including their ability to function as antigen-presenting cells and modulate the activation and differentiation of T-cells. In aim #1, we shall characterize a novel population of B-cells that we have found to be expanded in patients with SLE and will ascertain whether these cells represent abnormal expansions of either marginal zone B-cells or germinal center B-cells. Aim # 2 will elucidate the cellular origin of different types of autoantibodies with opposing behavior in terms of their spontaneous fluctuation and response to immunosuppressive therapy. Specifically, we shall test the hypothesis that anti-ds DNA antibodies are produced by short-lived plasma cells while anti-RNP antibodies are made by long-lived plasma cells. We will investigate which B-cell subsets constitute the precursors for each type of plasma cell and establish at which stage during plasma cell differentiation do the different types of autoantibody-producing B-cells segregate into the short-lived and long-lived compartments. Together, these aims will contribute to identify defective checkpoints in the maintenance of B-cell tolerance in human SLE. Furthermore, the information obtained will help identify cellular and molecular targets for the development of new therapeutic interventions. Finally, in aim # 3, we will study the role of regulatory T-cells in suppressing autoimmunity in the B-cell and T-cell compartments in normal subjects. Based on that information, we will determine whether regulatory T-cell function is defective in SLE. Given that similar studies are proposed in Projects 1 and 2 of our Center application for Type 1 Diabetes and Multiple Sclerosis, the combination of our efforts will substantially enhance current knowledge regarding the importance of this critical T-cell population in human health and autoimmunity.